There's also what else comes with these engines. The James deluxe adds milk & tar wagons, his own timetable and 4 James stories. I think each engine carries their own popularity ranking better than others, they all will have an audience and depends on what is offered. If we get one for Fergus and another for Billy, more will get Fergus. Depends what comes with them too, like if you have to get an engine you're not keen on just for a unique rolling stock that's been bundled with them (e.g. Belle with Water Tankers).
TSW's expansions come in at £20 to £30 for a route plus a couple of locomotives depending on the length of the route. WoS will almost certainly follow the same pricing model so I wouldn't get your hopes up otherwise. It already has done in terms of the James expansion which matches TSW's single locomotive add ons.
